The present invention is directed generally to an igniter structure and more particularly to an igniter of the type utilized to ignite a converter screen assembly which acts as a catalyst for vapor phase oxidation of ammonia in a nitric acid plant.
In a typical nitric acid plant, a pad or sheet of platinum-rhodium screens is used as a catalyst for vapor phase oxidation of ammonia in a part of the nitric acid reduction process. This screen assembly is heated to a sufficient degree to cause "burning" of the hydrogen in the ammonia with the oxygen in the air to produce an enriched nitrogen or "nitric oxide" product. This nitric oxide product is then further processed to become nitric acid.
Conventionally, the screen assembly or "gauze" is mounted in a generally horizontal orientation within a sealed chamber into which a mixture of ammonia and air at the proper temperature is fed. Thereupon, the screen or gauze must be heated sufficiently to cause ignition of the ammonia and air mixture. This ignition tends to occur in a limited area of the screen at first, whereupon the area of ignition gradually increases until ignition occurs substantially throughout the screen.
One significant problem in prior art arrangements has been to provide an effective igniter structure for initiating the ignition process in the screen or gauze. In one prior art arrangement this ignition was accomplished by means of a high resistance "glow wire" mounted on a pair of conductive rods which were rigidly mounted through the sealed chamber wall for delivering electrical power to the glow wire. However, in practice, the positioning of the glow wire with respect to the screen or gauze proved critical and very difficult to accomplish. Repositioning was accomplished by bending the mounting rods to the desired position, which required shutdown of the process for entry to the sealed chamber and subsequent re-sealing.
If the glow wire were positioned too far from the screen, ignition would not occur. On the other hand, if the glow wire were too close to the screen, vibrations caused by the air and gas flow in the chamber could cause the screen to strike the relatively fragile glow wire causing breakage of the wire and/or possible damage to the screen. Such breakage and damage require complete shut down of the process for expensive and time consuming repair of the screen and/or glow wire.
The foregoing structure was improved upon by replacing one of the electrode rods for the glow wire with a tube through which hydrogen gas was passed to convert the igniter to a small hydrogen torch. That is, the glow wire was utilized to cause ignition of the hydrogen, producing a relatively hot, concentrated flame for igniting the gauze or screen. However, breakage of the glow coil due to improper positioning too close to the screen or gauze still often occurred. Additionally, the relatively concentrated flame tended to impinge in a relatively small area of the screen or gauze. Hence, careful control of hydrogen delivery and pressure proved necessary. If the hydrogen pressure became too great or the flame was allowed to burn for too long a period, burning or melting of the screen or gauze and associated support structure would occur, again necessitating costly shutdown and repair.